Nearly 23 years after 9/11, a critical flaw that helped terrorists storm the cockpits of four jetliners, kill the pilots and turn the planes into weapons of mass murder is finally being corrected.

President Joe Biden on Thursday signed into law provisions requiring secondary cockpit barriers on all commercial airplanes – the last of the 9/11 Commission’s recommendations yet to be implemented.

The lightweight, lockable metal gates — long opposed by the airline industry because of their added cost of roughly $35,000 a pop — barricade cockpit doors to protect pilots when they open them in flight to use the restroom or receive food.

“I guess I wore down Congress enough that they were tired of me,” said Ellen Saracini, widow of Capt. Victor Saracini, a pilot on doomed United Flight 175, which was hijacked and smashed into the World Trade Center’s south tower.

“The airlines have had great lobbying efforts against this over the years, and I think they thought the little widow was going home in two weeks, but I decided not to,” she told The Post.

“It’s been a lot of work, a lot of years, but my commitment was to never let this happen again,” added Saracini. “Victor didn’t die in vain.”

The changes won’t happen overnight, however.

It may take more than five years for the airlines to retrofit roughly 8,000 airplanes with the new barriers. The feds have up to November 2028 to issue final regulations.

Biden ratified the provisions by signing off on a larger five-year, $105 billion civil aviation bill aimed at improving air travel. It was approved 387-26 by the House of Representatives Wednesday and 88-4 a week earlier by the U.S. Senate.

The bill included the Saracini Enhanced Aviation Safety Act, named after the late pilot, which was authored by Rep. Brian Fitzpatrick (R-Pa), and co-sponsored by Sen. Bob Casey (D-Pa).

A prior act passed in 2018 required secondary cockpit barriers on newly built planes. The amendment in the latest bill mandates the barriers on all existing passenger aircraft.

Praising Ellen Saracini’s “resilience and unwavering dedication,” Fitzpatrick said, “For years, we have fought together to ensure the horrific events of 9/11 are never able to happen again, and this win marks a pivotal advancement in fulfilling that critical mission.”

The hijackers sent by Al Qaeda leader Osama bin Laden brought down four planes on Sept. 11, 2001, two crashing into the World Trade Center, one into the Pentagon and one onto a field in Shanksville, Pa., after brave passengers fought to wrest control before it could reach the White House or U.S. Capitol. 

Victor Saracini, 51, was the father of two young daughters when he perished.

“In view of the high number of recent unruly passenger incidents, and the increased threat posture due to the Israeli-Hamas war, this long overdue protection against cockpit intrusions is certainly welcome,” said former Federal Aviation Administration Special Agent Brian Sullivan.

“No longer will flight attendants be forced to block the cockpit with their little beverage cart — a method that provided about three seconds of additional protection and put the flight attendants at risk,” he said.  

Angelo Roefaro, a spokesman for Senate Majority Leader Chuck Schumer (D-NY), said “the tragedy of 9/11 showed improvements were needed in both airport security and planes themselves,” but the “new [rule] will mark an additional and critical safety measure now complete.”

Rep. Nicole Malliotakis (R-Staten Island/Brooklyn) said it’s still not enough.

“It’s an important measure in preventing hijacks, but we also need more stringent regulations when issuing government IDs and to stop illegal immigrants from boarding airplanes with [arrest] warrants as their sole identification, as the [Transportation Security Administration] has admitted is occurring,” said Malliotakis.

“We need to take any and all precautions to ensure a terrorist does not exploit the Biden administration’s lax policies.”